Has anyone started a project similar to Grasshopper (Rhino) for blender?
Sverchok

There is such a tool, it is called Sverchok. It's russian for "Cricket" as a sign of respect to Grasshopper. Sverchok is a Blender add-on that works on all platforms, and has over 180 node types. 


The project github page: https://github.com/nortikin/sverchok/ (including download links)
Ask questions on the issue tracker


To get a sense of what people are making with Sverchok checkout the google+ group   


sverchok community